Related Field
The present disclosure relates to a method and system for rendering a Synthetic Aperture Radar image.
Description of Related Art
Synthetic-aperture radar, SAR, is a form of radar whose defining characteristic is its use of relative motion, between an antenna and its target region, to provide distinctive long-term coherent-signal variations, that are exploited to obtain finer spatial resolution than possible with conventional beam-scanning means.
SAR is usually implemented by mounting on a moving platform such as an aircraft or spacecraft, a single beam-forming antenna from which a target scene is repeatedly illuminated with pulses of radio waves. Echoes successfully received at the different antenna positions are coherently detected and stored. The stored echoes are then post-processed to resolve elements in an image of the target region.
SAR can also be implemented as “inverse SAR” by observing a moving target over time with a stationary antenna.
SAR's single physical antenna element gathers signals at different positions at different times. When the radar is carried by an aircraft or an orbiting vehicle, those positions are functions of a single variable, distance along the vehicle's path, which is a single mathematical dimension (not necessarily the same as a linear geometric dimension). The signals are stored, thus becoming functions, no longer of time, but of recording locations along that dimension. When the stored signals are read out later and combined with specific phase shifts, the result is similar as if the recorded data had been gathered by an equally long and shaped phased array.
The core of the SAR technique is that the distances that radar waves travel to and back from each scene element comprises some integer number of wavelengths plus some fraction of a “final” wavelength. Those fractions cause differences between the phases of the re-radiation received at various SAR or array positions. Coherent detection is used to capture the signal phase information in addition to the signal amplitude information. That type of detection requires finding the differences between the phases of the received signals and the simultaneous phase of a sample of the transmitted illumination.
In a typical SAR application, a single radar antenna is attached to an aircraft or spacecraft so as to radiate a beam whose wave-propagation direction has a substantial component perpendicular to the flight-path direction. The beam is allowed to be broad in the vertical direction so it will illuminate the terrain from nearly beneath the aircraft out toward the horizon.
Resolution in the range dimension of the image is accomplished by creating pulses which define short time intervals, either by emitting short pulses comprises a carrier frequency and sidebands, all within a certain bandwidth, or by using longer “chirp pulses” in which frequency varies, (often linearly), with time within that bandwidth. The differing times at which echoes return allow points at different distances to be distinguished.
The process can be thought of as combining the series of spatially distributed observations as if all had been made simultaneously with an antenna as long as the beam width and focused on that particular point. The “synthetic aperture” provided at maximum system range by this process not only is longer than the real antenna, but, in practical applications, it is much longer than the radar aircraft.
Combining the series of observations requires significant computational resources, usually using Fourier transform techniques. The high digital computing speed now available allows such processing to be done in near-real time on board a SAR aircraft. The result is a map of radar reflectivity, including both amplitude and phase. The amplitude information, when shown in a map-like display, gives information about ground cover in much the same way that a black-and-white photo does.
The two dimensions of a radar image are range and cross-range. Other terms used instead of cross-range are Doppler, azimuth, side etc. A regular Synthetic Aperture Radar transforms a 3D world into a 2D representation. The 2D representation exhibits cylinder symmetry at the imaging. Accordingly, it is not possible to discriminate objects having the same distance to the cylinder axis, i.e., the synthetic aperture, if they have the same position along the axis. The cylinder coordinates can be explained as the measuring coordinates in SAR. Cone coordinates can also be used (for example range and Doppler).
It takes some time and training getting used in order to correctly interpret SAR images. To assist in that, large collections of significant target signatures have been accumulated by performing many test flights over known terrains.
“Correction of Positional Errors and Geometric Distorsions in Topographic Maps and DEMs Using a Rigorous SAR Simulation Technique”, photogrammetric engineering & remote sensing, september 2004, pages 1031-1042, relates to detection and correction of positional errors and geometric distorsions in topographic data based on Synthetic Aperture Radar, SAR, image simulation and mathematical modelling of SAR imaging geometry.